The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.
The present invention relates to vapor phase epitaxy, and particularly, to a method of accurately and quickly varying the rate of flow of a gaseous component into a reaction chamber.
Vapor phase epitaxy is a well-known process for forming layers of semiconductor material on a substrate. Generally, the process directs gaseous reactants into a reaction chamber for deposition of the desired material onto a substrate. The operating temperature and pressure of the process is specific to the individual materials being combined, or deposited, and is well known in the art.
Often, it is desirable to add an additional impurity in the form of a gaseous reactant, such as a conductivity modifier, into the reaction chamber in order to form a layer of the semiconductor material in which the conductivity modifier is present. It is often necessary to quickly and accurately vary the rate of flow of at least one gaseous component into the reaction chamber in order to form semiconductor layers which sharply differ in concentration of that one component, or for other purposes, such as controlling the growth rate. For example, it is often desirable to form a sharp change in carrier type or carrier concentration through the process of vapor phase epitaxy. In order to form the desired layers as quickly as possible, it is necessary to minimize the time it takes to introduce particular gaseous components into the reaction chamber. Present methods attempt to minimize this introducing time by placing the source of gaseous components as near as possible to the reaction chamber. In addition, present methods often slow down the overall growth rate in order to compensate for the delay caused by the introduction of a small quantity or low flow rate of a gaseous component into the reaction chamber. A more drastic technique often employed involves stopping the overall growth rate and then adding a particular gaseous component into the chamber, or equally drastic, moving the substrate into another deposition chamber.
Although present methods are often adequate for steady state conditions, these methods are less than desirable when it is necessary to sharply and accurately introduce or regulate at least one gaseous component into the reaction chamber. Present methods are especially undesirable where it is necessary to accurately and quickly introduce or regulate a small quantity or low flow rate of gaseous component into the reaction chamber while other gaseous materials are forming the semiconductor layers in the reaction chamber, e.g., for forming very thin multilayer gallium arsenide structures for field effect transistors, Impatt diodes and other devices.
Present methods are unable to quickly and accurately vary the rate of flow of a small quantity of a gaseous component into the reaction chamber since the gaseous component will initially enter the reaction chamber in a large surge of concentration followed by a lower concentration. Furthermore, the small quantity or low flow rate of the gaseous component will also be delayed by the time necessary to travel from the controlling device to the reaction chamber. It would therefore be desirable to develop a method for accurately and quickly varying the rate of flow of a gaseous component into the reaction chamber.